The present invention resides in pay television systems and, more specifically, in such systems wherein a determined characteristic of the composite television signal is changed either periodically or aperiodically at the temperature prior to transmission and code signals are added to the signal to indicate which, if any, of a number of predetermined changes have taken place. Each receiver of the pay-television system has a decoder to process the received encoded signal (i.e. to decode), in accordance with a decoder mode set up to correspond to the transmitted code only after a charge has been properly entered for the program.
In known systems of the above-described type, it is possible that the decoder mode is improperly set and the subscriber is billed for a program which he does not actually receive or which is not properly decoded.